The mixing together of dissimilar fluids such as a gas and a liquid or two dissimilar liquids has many industrial applications. For example, the mixing of oxygen and a liquid has applications such as the oxygenation of water for biological purposes and the oxygenation of fuel prior to burning to enhance combustion efficiency. The mixing of air and a liquid may be used in the pulp and paper, textile, and other industries in a process known as dissolved air floatation to separate suspended particulate materials from the liquid. Mixing industrial stack gasses with a liquid such as water is useful for removing environmental contaminants from the stack gasses prior to its release to the atmosphere. The mixing of dissimilar fluids such as, for example, oil and water, has industrial application in the creation of emulsions. Further, an existing emulsion may be separated into its constituent components by mixing it with another gas or fluid such as, for example, methane, which acts as an inhibitor to prevent recombination of the constituent components once they are broken apart.
One particular industrial application of gas/liquid mixing occurs in the pulp and paper industry where black liquor, which is a byproduct of cooking wood chips, often is recycled by being burned as a fuel in boilers. Even though such recycling is economically efficient, an emissions problem arises from the fact that untreated black liquor contains concentrations of Sodium Sulfide (Na2S) that can be as high as 40 grams per liter or more. As a result, when untreated black liquor is burned, the Sodium Sulfide contained therein is converted to Sodium Dioxide (SO2) and Hydrogen Sulfide (H2S), which are known as totally reduced sulfur (TRS) compounds. TRS compounds are extremely harmful to the environment and therefore are highly regulated and may not be released to the atmosphere as a component of boiler stack gasses. Accordingly, black liquor often is treated before being burned in order to reduce or eliminate TRS emissions.
One method of treating black liquor prior to burning has been to mix or agitate it with air in a gas/liquid mixing process. When so mixed, Sodium Sulfide within the black liquor is oxidized in a chemical oxidation/reduction or “redux” reaction with oxygen molecules in the air and thereby converted to Sodium Thiosulfate (Na2S2O3). Unlike Sodium Sulfide, Sodium Thiosulfate exists in a stable chemical state and thus does not participate in chemical reactions when the treated black liquor is burned in a boiler. Instead, the Sodium Thiosulfate simply precipitates to the bottom of the boiler, where it is ejected as a smelt.
Prior art industrial methods of mixing gases and liquids in general, and air and black liquor in particular, have involved introducing air in the form of bubbles into black liquor and agitating the mixture to break up the air bubbles and distribute them throughout the liquor. The goal, of course, is that oxygen molecules in the air will react chemically with or “oxidize” sodium sulfide molecules in the black liquor, rendering them inert during combustion of the black liquor. In one prior art process, such mixing is accomplished with rotating mechanical beaters having blades that impact and cut up the air bubbles while agitating the liquid. The problem with such a system, however, is that there is a natural lower limit to the size of the resulting air bubbles because larger bubbles cannot be cut or chopped to a size smaller than the size of the beater blades. Thus, the total composite surface area of the air bubbles in contact with the black liquor is severely limited. As a result, the probability that an oxygen molecule within an air bubble will come into contact with and oxidize a Sodium Sulfide molecule within the black liquor is reduced.
A further exacerbating problem and limitation of prior art gas/liquid mixing methods in general, and black liquor oxidation processes in particular, arises from the fact that the bubbles that are created by the mechanical beater blades of the mixing apparatus tend not to be distributed evenly throughout the black liquor. Instead, the bubbles, partially because of their relatively large size and partially because of the mechanical nature of the process, tend to agglomerate or concentrate into pockets of bubbles separated by relative voids in the liquor. This further reduces the probability that an oxygen molecule within an air bubble with come into contact with or “find” a Sodium Sulfide molecule and thus reduces the efficiency of the oxidation process. To address this inefficiency, it may be necessary to inject many times the amount of air necessary to oxidize the Sodium Sulfide into the mixer and to increase mixing times substantially to increase the probability of oxidation. However, such a brute force method of increasing oxidation efficiency substantially increases the time, energy, and resources required in the mixing process and thus introduces its own inefficiencies.
A final limitation of prior art gas/liquid mixing methods as applied to the oxidation of black liquor is imposed by the fact that the molecules within the liquor are attracted to each other by weak molecular forces known as van der Waals attraction. This results in the molecules clumping together in mutually attracted groups. In many cases, a Sodium Sulfide molecule that needs to come into contact with an oxygen molecule within a bubble in order to be oxidized may be surrounded within such a group by other molecules within the liquid and thus shielded from contact with a bubble and an oxygen molecule. In these cases, oxidation of the Sodium Sulfide molecule can not occur regardless of the volume of gas introduced or the length of the mixing process. This is due, in part at least, to the fact that the energy imparted to the liquor by mechanical beater blades is far less than that required to break the van der Waals attractions and free trapped molecules. In effect, then, the molecular Van der Waals attraction within the liquor imposes a physical limit to the percentage of Sodium Sulfide molecules within black liquor that can be oxidized with traditional gas/liquid mixing techniques.
Thus, a specific need exists for a gas/liquid mixing method and apparatus applicable to the oxidation of black liquor in the pulp and paper industry that overcomes the problems, shortcomings, and limitation of prior art processes. More generally, a need exists for a new and unique method of mixing dissimilar fluids, be they gasses and liquids, dissimilar liquids, or otherwise, that is highly efficient, that results in virtually complete mixing in a short time and with a minimum of required energy and resources, and, in the case of oxidation applications, overcomes the physical limits on oxidation efficiency imposed by molecular van der Waals attraction. It is to the provision of such a method and an apparatus for carrying out the method that the present invention is primarily directed.
For clarity of disclosure and discussion, the present invention will be discussed herein primarily in the context of its application to the oxidation of environmental contaminants such as Sodium Sulfide in black liquor within the pulp and paper industry. Such an application is considered by the inventors to be a best mode of carrying out the invention. It will be understood and appreciated, however, that the method and apparatus of the invention is applicable to virtually any situation where dissimilar fluids are to mixed together for industrial or commercial purposes. For example, the invention is applicable in the pulp and paper industry alone to a variety of processes including micro-mixing prior to gasification, mixing stack gases with black liquor in direct contact evaporators, mixing salt cake and black liquor, pulp drying, sludge dewatering, oxygen de-lignification, pulp bleaching by mixing pulp with ozone or other appropriate gases, and atomization of black liquor prior to its use in a recovery boiler. In the petroleum industry, the invention is applicable among other things to the separation of tight emulsions using micro-mixing and to heavy oil upgrading. Within the food processing industry, the mixing methodology of the invention has application in homogenization, oxygenation, and spice mixing processes. Applications within the environmental industry include oxidation/reduction of liquids or components of liquids, concentration and evaporation, BOD and COD reduction, dissolved air floatation, and fuel aeration. Thus, the discussion of the invention herein within the context of black liquor oxidation should not be interpreted as a limitation of the invention but only as representing a preferred embodiment or application and a best mode of carrying out the invention.